As positive type photoresist compositions there are normally used compositions comprising an alkali-soluble resin and a naphthoquinone diazide compound as light-sensitive material. Examples of such compositions include novolak type phenol resin/naphthoquinone diazide-substituted compounds disclosed in U.S. Pat. Nos. 3,666,473, 4,115,128, and 4,173,470. Most typical examples of such compositions include novolak resin made of cresol-formaldehyde/trihydroxybenzophenone-1,2-naphthoquinonediazidesulfon ic ester disclosed in L. F. Thompson, "Introduction to Microlithography", ACS, No. 219, pp. 112-121.
A binder, novolak resin can be dissolved in an alkaline aqueous solution without swelling. The novolak resin can also exhibit a high resistance particularly to plasma etching when an image thus produced is used as a mask for etching. Thus, novolak resin is particularly useful in this application. A light-sensitive material, naphthoquinone diazide compound itself serves as a dissolution inhibitor for reducing the alkali solubility of novolak resin but is peculiar in that it undergoes decomposition upon the irradiation with light to produce an alkali-soluble substance which rather enhances the alkali solubility of novolak resin. Because of the great change in properties by the irradiation with light, naphthoquinone diazide compound is particularly useful as light-sensitive material for positive type photoresist.
From such a standpoint of view, many positive type photoresists comprising novolak resin and naphthoquinone diazide light-sensitive material have heretofore been developed and put into practical use. These positive type photoresists have attained sufficient results in working lines of a width of 1.5 .mu.m to 2 .mu.m.
However, integrated circuits have added to their degree of integration. It is a recent tendency that the working of ultrafine patterns formed of lines of a width of 1 .mu.m or less is required in the production of semiconductor substrates such as SLSI. In such an application, a photoresist having a high resolution, a high accuracy of reproduction of exposure mask pattern and a high sensitivity for high productivity has been desired.
In such an application, techniques concerning monomer compositions and synthesis methods have been disclosed. For example, JP-A-62-35349 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses the use of a novolak resin obtained by the addition condensation of a cresol mixture consisting of 10 to 45% by weight of m-cresol and 55 to 90% by weight of p-cresol with formaldehyde in the presence of an acid catalyst. However, this method is disadvantageous in that when it is attempted to obtain a high resolution and an excellent profile, the resulting sensitivity is low.
Further, JP-A-60-159846 discloses the use of a novolak resin obtained by the addition condensation of phenols with formaldehyde at a pH value of 4 to 7 in the presence of an organic acid salt of a divalent metal electronegatively lower than hydrogen as catalyst. This process, too, is disadvantageous in that although it provides an excellent sensitivity and resolution, it gives a poor heat resistance.
Thus, none of these approaches can provide satisfactory properties.